AN1049 datasheet(7/24 Pages) STMICROELECTRONICS

7 / 24 page

The inspection of equations (15)...(23) shows that also in MCM systems the effect of the offset Vo is the

same as in DCM systems. Furthermore, the internal thresholds VT1 and VT2 are such that a large range

of applications can be covered without any external adjustment.

Standby function setup

It is difficult to outline a general procedure

for the use of the L5991’s standby function

because the constraints of a specific design

may be of different types and are not known

in advance. It is possible, however, to pro-

vide some diagrams that summarize the

analysis previously carried out and that can

be used for reference.

In figure 5 the ratio PinSB/Pinmax is plotted

against the offset voltage on current sense

Vo, for different values of the parameter KM

defined as:

Pinmax

PinTmin

(24).

In figure 6, the ratio PinNW/Pinmax is plotted

against the ratio fosc / fSB for the two ex-

treme values (0 and 200 mV) considered

for Vo.

The inspection of such diagrams shows a

large influence of Vo on PinSB, but a much

smaller influence on PinNW, which depends

mainly on the ratio fosc / fSB. If the values of

fosc and fSB are both already fixed, there is

little room for the adjustment of PinNW. This

is not usually a problem because there is no

harmful effect if the converter is operating

at fsw = fSB even when the load is not so

light (e.g. 40% of the maximum load or

even more).

This considering, one possible step-by-step

procedure could be the following:

1. Check whether the flyback is DCM or

MCM. To this end, from table (A1) pick

up the value of VEmin relevant to the

specification value and calculate IppkTmin:

IppkTmin

VEmin

⋅ fosc

If the resulting value is greater than 1/Rs

then the system will be DCM, otherwise

MCM.

2. Calculate Pinmax. If the system is DCM

use the following equation:

Pinmax

⋅ Lp ⋅ 









⋅ fosc

(DCM)

otherwise use:

Pinmax

VEmin

−

VEmin

⋅ Lp ⋅ fosc

(MCM).

Vo [mV]

KM = 3

≤ 1

KM = 2.5

KM = 2

KM = 1.5

PinSB

Pinmax

KM =

Pinmax

PinTmin

100

150

200

Figure 5. PinSB /Pinmax ratio vs. DC offset on current

sense.

2.5

3.5

4.5

)

01 z

)

0 1.5 z

)

02 z

)

0 2.5 z

)

03 z

KM = 3

≤ 1

KM = 2.5

KM = 2

KM = 1.5

PinNW

Pinmax

fosc

fSB

Vo = 0

2.5

3.5

4.5

)

200 1 z

)

200 1.5 z

)

200 2 z

)

200 2.5 z

)

200 3 z

KM = 3

≤ 1

KM = 2.5 KM = 2

KM = 1.5

fosc

fSB

Vo = 200 mV

PinNW

Pinmax

Figure 6. PinNW / Pinmax ratio vs. fosc / fSB ratio for 0

and 200 mV DC offset on current sense.

AN1049 APPLICATION NOTE

7/24

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